Introduction

Poisonous plants represent one of the greatest hazards to livestock ranching on western rangelands. In addition to direct losses by death, there are those due to weight loss, poor reproductive performance, and poor health. Through scientific reports and practical experience, stockmen have learned to recognize poisonous plants and keep livestock losses to a minimum. Some of the things that can be done to prevent problems with poisonous plants are:

Know the poisonous plants. This is the first line of defense against toxic plants.

Practice good range management. To a limited degree, poisonous plants will always be present on rangelands, but the real danger is when the plants are eaten. Livestock poisoning is more often the result of poor range condition and poor management than the presence of poisonous plants. Most poisonous plants are unpalatable and will be eaten only if other good quality forage is not available.

Feed adequate mineral and vitamin A supplements. Livestock in poor health are more susceptible to many toxins than are healthy animals.

Provide adequate forage, especially during early spring or drought periods.

Avoid driving livestock through infested or poor condition range. Hungry animals lose their selectivity, and poor rangelands contain far more poisonous plants than those in good condition.

The most common toxins in New Mexico poisonous plants are alkaloids, glycosides, minerals, nitrogenous compounds, oxalates and other organic acids, compounds causing photosensitivity, and resins or resinoids (see Tables 1 and 2).

Table 2. Distribution of Toxins in the Major Plant Families in New Mexico

Family
Toxin

Number of species
in which toxin occurs
in New Mexico

Family
Toxin

Number of species
in which toxin occurs
in New Mexico

Fabaceae

Ranunculaceae

Alkaloids

21

Alkaloids

6

Dicoumarin

2

Glycosides

4

Glycosides

3

Hairballs

1

Photosensitization

3

Irritant oils

1

Saponin

1

Apiaceae

Selenium

13

Alcohol

2

Unknown

10

Alkaloids

2

Asteraceae

Nitrogenous compounds

2

Acrid substance

2

Unknown

2

Alcohol

1

Brassicaceae

Alkaloids

5

Allylisothiocyanate

2

Glycosides

3

Glycosides

1

Nitrogenous compounds

2

Mustard oil

1

Photosensitization

2

Selenium

1

Saponin

2

Unknown

2

Selenium

11

Ascelepiadaceae

Unknown

21

Alkaloids

7

Volatile oils

2

Glycosides

7

Poaceae

Resinoids

7

Fungus

1

Rosaceae

Glycosides

3

Glycosides

7

Mechanically injurous

10

Euphorbiaceae

Nitrogenous compounds

3

Acrid substance

3

Photosensitization

2

Croton oil

1

Unknown

2

Photosensitization

1

Solanaceae

Unknown

1

Alkaloids

11

Liliaceae

Glycosides

1

Alkaloids

4

Unknown

2

Unknown

2

Chenopodiaceae

Polygonaceae

Antihelminthic oil

1

Nitrogenous compounds

1

Glycosides

1

Oxalates

4

Nitrogenous compounds

3

Photosensitization

1

Oxalates

4

Photosensitization

1

Selenium

3

Alkaloids. These are the most powerful poisons and produce strong physiological reactions, generally affecting the nervous system. The alkaloid is usually distributed throughout the plant and is active whether it is fresh or dry. There are no antidotes for alkaloid poisoning. Some of the common alkaloid-containing range plants are locoweeds (Astragalus spp.), larkspur (Delphinium spp.), groundsel (Senecio spp.), and lupine (Lupinus spp.).

Glycosides. Glycosides are compounds composed of a toxic agent combined with a sugar. The toxic agent is harmful when released from the glycosides. Cyanide (HCN, prussic acid) is the most common toxin in the glycosides. Glycoside levels in the plant are often related to environmental factors such as freezing, wilting, drought, and crushing. Cyanide is generally not retained in plant tissues after drying. Catclaw (Acacia greggii), chokecherry (Prunus virginiana), and johnsongrass (Sorghum halepense) are potential cyanide producers.

Minerals. Excessive absorption of certain minerals may be dangerous to livestock. Selenium is possibly the most important poisonous mineral in New Mexico. It may be found in some species of locoweed (Astragalus spp.), fourwing saltbush (Atriplex canescens), snakeweed (Xanthocephalum sarothrae), and asters (Aster spp.). Prince’s plume (Stanleya pinnata) grows only on seleniferous soils.

Nitrogenous compounds. Both range and crop plants may contain potentially toxic levels of nitrogenous compounds. Care must be taken during drought or after fertilizer or herbicide application. Nitrates are also toxic in dry material and silage. Nitrogenous compounds often are found in many species of the amaranth (Amaranthaceae), sunflower (Asteraceae), mustard (Brassicaceae), and gooscfoot (Chenopodiaceae) families.

Oxalates and organic acids. Many plants contain oxalates, but few species in New Mexico contain enough to be considered dangerous. Halogeton (Halogeton glomeratus) and dock (Rumex spp.) possess soluble oxalates that are corrosive to animal tissue. Non-ruminants are more susceptible to oxalate poisoning than ruminants because the large rumen capacity enables the animal to detoxify small amounts of soluble oxalates. Most cases of ruminant poisoning occur on poor condition rangelands.

Compounds causing photosensitivity. On certain occasions, animals become hypersensitive to light. Photodynamic pigments contained in horsebrush (Tetradymia spp.), lechuguilla (Agave lechuguilla), sacahuista (Nolina spp.), and puncture-vine (Tribulus terrestris) react with light to produce swelling about the head, blistering, and a sunburned appearance. Damage may be so severe that the lips, ears, and eyelids may be lost. Bacterial infection may also set in.

Resins or resinoids. These are highly complex organic compounds that are present in a number of plants. Resins are found in milkweeds (Asclepias spp.), marijuana (Cannabis sativa), water hemlock (Cieuta douglasii), and chinaberry tree (Melia azedaraeh).

Several noxious plants contain no poisonous compounds, but if grazed, cause injury through mechanical means. This is often by long awns or bristles (Aristida spp., Hordeum jubatum,Stipa spp.) or by stinging hairs (Urtiea spp.).

Poisonous Plants of New Mexico

There are at least 270 poisonous plants known to occur in New Mexico (Table 3). The following list includes all New Mexico range plants known to be poisonous or injurious to livestock or man, even though many of these do not pose a threat except in exceptional circumstances. Also included are a few very common poisonous ornamentals or garden plants that may occasionally escape. The major poisonous plants that are likely to cause problems and should be watched for are marked with an asterisk (*). The list is arranged in alphabetical order by plant family and scientific name. Indications of toxins and animals affected are only those reported and verified. The plants may be toxic to animals other than those listed.

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